1). Field of the Invention
This invention relates to a method of forming a silicon nitride layer on a substrate.
2). Discussion of Related Art
Integrated circuits are often formed in and on a semiconductor wafer substrate. One or more steps may include the formation of a silicon nitride layer on a surface of the wafer substrate. The surface of the wafer substrate often has alternating raised transistor gates and low regions between the transistor gates. Very narrow low regions may be formed between some of these formations, and the surface may also have entirely flat larger areas.
A mixture of silane (SiH4) gas and ammonia (NH3) gas is often used to form the silicon nitride layer over such a surface. The silicon nitride layer is subsequently etched back to form spacers adjacent to the transistor gates. The SiH4 gas and the NH3 gas are introduced into a processing chamber and react with one another to form silicon nitride, which deposits on the surface. The rate at which the silicon nitride layer forms depends on the processing conditions, in particular the pressure within the chamber. A higher pressure will result in a higher rate at which the silicon nitride layer forms. In one process, the pressure is maintained relatively low, but many wafers are simultaneously processed in one chamber so that a relatively high throughput can be maintained.
Should a single wafer be processed in a chamber, it may be required to increase the pressure to obtain a higher rate at which the silicon nitride layer forms. It has, however, been found that substantial variations in thickness result when a silicon nitride layer is formed at a high rate and pressure, in particular when comparing deposition rates, and corresponding resulting thicknesses, between larger flat areas and narrow low regions between transistor gates.
This invention relates generally to a method of forming a silicon nitride layer on a substrate. A substrate having a surface with raised transistor gates and low regions between the transistor gates is inserted into a processing chamber. The substrate is heated to a processing temperature. A pressure within the chamber is maintained at a processing pressure. A silicon-containing gas is introduced into the chamber. NH3 gas is introduced into the chamber. The silicon-containing gas and the NH3 gas react with one another to form a silicon nitride layer on the surface. The substrate is removed from the chamber when the silicon nitride layer is formed on the surface.
The silicon nitride layer is formed while the processing temperature is relatively high, typically at least 500xc2x0 C., and the pressure is relatively high, typically at least 50 Torr, to obtain a relatively high rate of formation of the silicon nitride layer. Processing conditions are controlled so as to more uniformly form the silicon nitride layer. Generally, the ratio of the NH3 gas to the silicon-containing gas by volume is selected sufficiently high so that, should the surface have a low region between transistor gates which is less than 0.15 microns wide and have a height-to-width ratio of at least 1.0, as well as an entirely flat area of at least 5 microns by 5 microns, the layer forms at a rate of not more than 25% faster on the flat area than on a base of the low region. The ratio is generally between 200 and 350, although a much lower ratio may be possible if other processing conditions, in particular pressure, are controlled, with similar results.